Carbon fibers are produced commercially from rayon, phenolics, polyacrylonitrile (PAN), or pitch. The latter type are further divided into fiber produced from isotropic pitch precursors, and those derived from pitch that has been pretreated to introduce a high concentration of carbonaceous mesophase. High performance fibers, i.e. those with high strength or stiffness, are generally produced for PAN or mesophase pitches. Lower performance, general purpose fibers are produced from isotropic pitch precursors. These materials are produced as short, blown fibers (rather that continuous filaments) from precursors such as ethylene cracker tar, coal-tar pitch, and petroleum pitch prepared from decant oils produced by fluidized catalytic cracking. Applications of isotropic fibers include: friction materials; reinforcements for engineering plastics; electrically conductive fillers for polymers; filter media; paper and panels; hybrid yarns; and as a reinforcement for concrete.
More recently, interest has developed in activated forms of isotropic carbon fibers, where high surface areas can be produced by partial gasification in steam or other oxidizing gases. Activated carbon fibers have novel properties that make them more attractive than conventional forms (powders or large-size carbons) for certain applications. While porosity can be generated in most types of carbon fiber, low modulus fiber produced from isotropic pitch are particularly suited for activation because of their unique structure, where the random packing of small crystallites allows the development of an extensive pore structure.
Among the possible applications, activated carbon fibers are of interest for the adsorption and recovery of organic vapors; in environmental protection; the removal of SO.sub.x and NO.sub.x from flue gas; the improvement of air quality; and water treatment. Difficulties in handling and utilizing activated carbon fibers can be surmounted by their incorporation into composites, such as woven and non-woven fabrics, felt and paper. This invention provides a rigid, activated carbon fiber composite material that has an open and permeable structure and can be produced in single pieces to a given size and shape. The unique properties of the activated composite made from isotropic pitch derived carbon fibers, such as; narrow, unimodal pore structure in the fiber, high-surface area, rapid rates of adsorption and desorption, the ability to form specific shapes of high permeability and strength, suggest that, among other applications (notably in environmental protection), they may be suitable for molecular separation on the basis of size and shape.
Pressure swing adsorption (PSA) is a known process for separation of gases from multicomponent gas mixtures. The PSA apparatus relies on the physical process of adsorption, in which gases are selectively adsorbed onto a substrate from a gas stream, thus depleting the stream of one gaseous species. The adsorbed gas is then desorbed at a lower pressure into a second gas stream thus enriching it with the desorbed species. The desorption step regenerates the adsorbent material for reuse during the subsequent adsorption step. It is widely acknowledged that PSA technology has fully matured and that further advances in this technology will require the development of superior adsorbent and molecular sieve materials which have significantly higher surface areas combined with mean micropore width of 5-10 .ANG..
PSA systems typically comprise several adsorption beds, through which the gas stream is passed, allowing for the near complete separation of the selected gas species. The adsorbent materials used in a PSA unit are selected to have the appropriate mean micropore width (typically in the range of 5-10 .ANG.) to selectively adsorb or sieve the required gas species and additionally must possess large surface areas. Currently available adsorbent materials include zeolites, with surface areas in the range of 10-350 m.sup.2 /g, and activated carbons with surface areas in the range of 500-1000 m.sup.2 /g. Conventional activated carbons and carbon molecular sieves are granular in structure. During operation in a PSA system, granular materials suffer attrition and can settle resulting in the formation of channels which allow the fluid stream to bypass the adsorbent.
Therefore, it is an object of this invention to provide an activated carbon fiber composite material which comprises activated carbon fibers with controlled porosity.
It is another object of the present invention to provide an activated carbon fiber composite material in a rigid, monolithic form.
It is yet another object of the present invention to provide an activated carbon fiber composite which defines an open and permeable structure.